Applications Of Sm And CNTs In Al-Si Alloys And Their Performances

Al-Si series alloys, a group of traditional industrial aluminum alloys, are being widely used due to their low cost of raw materials,good castability, welding performance and wear resistance. ZL101 and ZL102 were used as the base alloys. However, their microstructures mainly consist of coarse dendrites of α-Al, polygonal primary Si blocks and needle-like eutectic silicon. In this paper, rare earth Sm and carbon nanotubes(CNTs) were used as modifier/reinforcement in Al-Si alloy. Then their microstructures and mechanical properties were studied.The results indicate that Sm can reduce the eutectic temperature. The greater Sm addition is, the more the eutectic temperature decreases.The eutectic temperature gets to its lowest as 0.6wt.% Sm added. A low eutectic temperature is a feature of the well-modified microstructure. As for microstructures, Sm can reduce the secondary dendrite arm spacing(SDAS) of α-Al, which reduces from 40μm to 27μm as with 0.6wt.% Sm addition in ZL101 alloys. Meanwhile, with proper addition of 0.6 wt.% Sm in ZL102, the primary α-Al phases appear as a slightly dual dendriticcellular and number of dendrites increase. In addition, Sm can modify the eutectic silicon from a coarse plate-like morphology to a fine fibrous one when the addition of Sm reached 0.6 wt.%. When proper amount of Sm is added into ZL102, simultaneous primary Si refinement and eutectic modification can be achieved. Moreover, the tensile properties and elongation are improved by the addition of Sm. And a excellent combined mechanical properties are obtained when the Sm addition is up to 0.6 wt.%.CNTs are added into base alloy as reinforcement to bring a qualitative leap in composites properties due to the high toughness and structural stability of CNTs. Nano-sized CNTs with low density, large specific surface area are easy to agglomerate and the wettability between Al alloy matrix and CNTs is very poor. To solve these problems, Al-CNTs master nanocomposites with a large concentration of CNTs are fisrt fabricated as CNTs carriers. Then they are diluted to fabricate CNTs/ZL101 nanocomposites with a lower concentration of CNTs by stir casting/ ultrasonic processing. The more CNTs content is, the longer time it will need for melting of Al-CNTs master nanocomposites. The best CNTs content is 5~8wt.%. The melting time can be shortened when Al-CNTs master nanocomposites were hot extruded. Mechanical stirringThe additions of CNTs can refine microstructures of CNTs/ZL101 nanocomposites which were fabricated by stir casting. However, CNTs dispersal capacity of mechanical stirring is limited. When the amount of CNTs exceed 0.4wt.%, there will be plenty of defects in nanocomposites, such as CNTs agglomerations, which result to degradation of mechanical performance of nanocomposites. The 0.2 wt.% CNT/ZL101 nanocomposites exhibit the highest Vickers hardness and UTS, which are increased by 36.8% and 18.8% when compared with the ZL101 matrix.Due to ultrasonic cavitation effect, CNTs agglomerations were broken down and uniformly dispersed in CNTs/ZL101 nanocomposites which were fabricated by ultrasonic processing. The eutectic silicon and α-Al were also refined.The microstructures of nanocomposites were completly refined when ultrasonic power reached 2.1kw. In the meantime, nanocomposites exhibit the highest Vickers hardness, UTS and elongation, which are increased by 22.8%, 17.8% and 47.2% respectively, when compared with the nanocomposites subjected by 0.7kw ultrasonic power. In addition, Vickers hardness and UTS of CNTs/ZL101 nanocomposites were increased with increasing additions of CNTs. The 1.2wt.% CNT/ZL101 nanocomposites exhibit the highest Vickers hardness(HV106.3) and UTS(267MPa), which are increased by 36.1% and 19.7%, when compared with the ZL101 matrix.